Amphocarboxylpropionate surfactants are amphoteric compounds that are the reaction products of 2-alkyl imidazoline, methyl acrylate, alkali metal hydroxide, and water. They are sodium chloride free. Therefore, the surfactant of its own accord will build or reduce the viscosity of a formulation. This then allows the formulator the option to control viscosity with whatever he chooses, without having to be concerned about the level of salt in the surfactant and the subsequent influence on viscosity building. They are excellent hydrotroping agents and are very useful as low irritancy additives in personal care formulations, such as baby shampoos, hand soaps, and dish soaps.
Amphocarboxylpropionate surfactants are commonly sold as a 40:54:6 (by weight) mixture of amphoteric:water:methanol. The methanol is present as a saponification by-product. In some cases it can be removed and in other cases it is removed and substituted with another alcohol such as ethanol, isopropanol, and propylene glycol. Sodium acrylate is also present as the result of saponified unreacted methyl acrylate and usually constitutes about 4-8% of the non-aqueous portion of the product. It is an undesirable by-product and adds nothing to a formulation.
Transporting and storing a product containing such a low concentration of active, however, is expensive and inconvenient.
Mixtures of 72:19:9 wt. % amphoteric:water:methanol and amphoteric:water:propylene glycol are also commercially available. However, they are highly viscous and tend to gel, both in their concentrated form and when added to an aqueous medium at temperatures lower than 70.degree. C. If not mixed during storage, a thick gelatinous material forms on the surface.
There is therefore a need for an amphocarboxylpropionate surfactant that can be highly concentrated, so as to save costs associated with storage and shipping, which is low in residual metal acrylate. In addition, there is a need for concentrated aqueous amphocarboxylpropionate solutions that do not gel when stored in the absence of stirring, or when diluted with water.